Gravity anomaly simulation
Alan Levine
ASU Geology 1987

snapshot of program

This program is very crude, interface wise. It was written in FORTRAN on and for a Mac Plus. It does not use the mouse- all input are via the keyboard.

However, it does teach students about the way different shaped anomalous bodies or ones of different densities could yield the same gravity curve.

This program will allow the you to create different subsurface bodies in order to match a given gravity anomaly pattern. As you know there are an infinite number of mass distribution and body geometries that will cause the same local variation in gravitational attraction. The anomaly is calculated for a user specified polygon based on the method of Talwani et al , 1959.

To run the program select the application gravity apl. You will always see three rectangular boxes. The top one will prompt you for input, the middle box will display the anomaly, and the bottom one is the X,Z coordinate system where you will build the anomalous masses (Z=depth, positive downward).

You will first be asked if you would like to start a new body or work on a previously created one stored as a file. When creating a new body you will be asked for:

  1. a filename for your data (less than 10 characters)
  2. your choice of 1 out of 3 mystery anomaly curves
  3. an initial density for the body (you can change this later)

You will then see displayed an initial rectangular body to start with. From this point you will be transferred to the vertex modifying routine. A grid is plotted to make coordinate selection easier and the vertices are numbered. It is crucial to the algorithm that the vertices are numbered in clockwise rotation.

Within the modifying routine you have four options. Add will let you plot more vertices. You must choice a numbered vertex after which the new one will be inserted and specify the coordinates of the new point. To remove a vertex choose Delete which will get rid of the vertex you indicate. Move enables you to pick a vertex and change its coordinates. The fourth option returns you to the main menu.

If you had chosen to work with old data your polygon and the anomaly curve to match will be displayed and you will be given the main menu.

From the main menu you can modify vertices as described above, move the entire polygon by translation in the X and/or Z direction, or change density of the body. Plot anomaly will calculate the gravitational attraction as for your anomalous body. The value of the anomaly is indicated by a horizontal cross bar atop a vertical line from 0 mgal; if the line has no cross bar, then the value is off the scale.

When you are ready to leave the program choose quit from the main menu. You will then be asked if you would like to save the data or ignore any changes you had made to previously created data.

Download the gravity program 54k, (Mac only)

Talwani, M, Worzel, JL, and Landisman, M, 1959,
Rapid Gravity Computations for Two-Dimensional Bodies with Application to the Mendocino Submarine Fracture Zone, Journal of Geophysical Research, 64:49-61.